Why a Weak Brief Wastes the Whole Team's Work
A brief is the cheapest tool you have for saving weeks of work. Here is what it needs to actually do that job.

Every time a designer delivers work and the client says "that is not what I meant", the problem is usually not the designer. It is a brief that got written in two minutes.
The whole team pays for a weak brief
Revision after revision, clarification meetings, and frustration on both sides. The time you "saved" writing the brief gets paid back multiplied by the size of the team.
What every brief needs
A clear objective: what should change after this work? Audience: who exactly are we talking to? Message: the one sentence that must land. References: examples close to what you want. And a feedback loop: who reviews, and when.
A simple test
A simple test: read the brief and ask yourself, so what do I do now? If there is no answer, send it back. Whoever receives it should be able to start working without a clarification meeting.